Imgui: Error in text width calculation?

I assume that text rendered as a whole (i.e. by calling ImGui::TextUnformatted("text")) and symbol by symbol like this:

ImGui::TextUnformatted("t");
ImGui::SameLine(0, 0);
ImGui::TextUnformatted("e");
ImGui::SameLine(0, 0);
...

should give the same result. However, ImGui::CalcTextSize does some substraction that does not seem to be right.
This demonstration is copied from opengl_example. It contains two functions to output text: as a whole and symbol by symbol. They give different results. First string is rendered as a whole and second string is rencder symbol by symbol.

However, if we change ImGui::CalcTextSize like this:

ImVec2 ImGui::CalcTextSize(const char* text, const char* text_end, bool hide_text_after_double_hash, float wrap_width)
{
    ImGuiContext& g = *GImGui;

    const char* text_display_end;
    if (hide_text_after_double_hash)
        text_display_end = FindRenderedTextEnd(text, text_end);      // Hide anything after a '##' string
    else
        text_display_end = text_end;

    ImFont* font = g.Font;
    const float font_size = g.FontSize;
    if (text == text_display_end)
        return ImVec2(0.0f, font_size);
    ImVec2 text_size = font->CalcTextSizeA(font_size, FLT_MAX, wrap_width, text, text_display_end, NULL);

    // Cancel out character spacing for the last character of a line (it is baked into glyph->XAdvance field)
//    const float font_scale = font_size / font->FontSize;
//    const float character_spacing_x = 1.0f * font_scale;
//    if (text_size.x > 0.0f)
//        text_size.x -= character_spacing_x;
    text_size.x = (float)(int)(text_size.x + 0.95f);

    return text_size;
}

two functions render the same result:

Tested on Ubuntu 16.04 64 bit.